The invention relates to a peristaltic fluid pump having a suction side and a pumping side and of the kind that comprises a pump housing having a mainly arcuate support surface, a flexible tube extending along this surface, a rotor having two opposite rollers for during operation rolling over the flexible tube along an entrance section where the tube successively is compressed, a pumping section extending across an angle of an arc of less than 180xc2x0, and an exit section where the compression successively is ended, whereby both sections is having an idling zone, respectively, without pumping action and a pumping zone, respectively, with pumping action, means for during operation making the rotor rotate, that the arcuate support surface is constructed in such a way that the two opposite rollers do not operate in synchronous phase opposition during operation.
Such a peristaltic fluid pump is used among other things for hemodialysis of a patient with kidney insufficiency, the pump thus serving for pumping the patient""s blood through a dialyzer.
Conventionally, a blood pump is arranged in such a way that both rollers can simultaneously be in pumping engagement with the flexible tube during operation. Thereby, the blood enclosed in the tube section between the rollers is subjected to a considerable positive pressure in relation to the wanted discharge pressure of the pump. When the leading roller is disengaged again from the tube, the positive pressure is relieved. The positive pressure and the compressive pulsations result in hemolysis of the red blood cells.
U.S. Pat. No. 3,787,148 discloses a peristaltic fluid pump having an arc-shaped bearing surface extending across a length of arc shorter than 180xc2x0 and at the ends passing into symmetrically arranged, short ramps extending obliquely in towards the arc-shaped bearing surface. In the application, it is stated that one of the rollers thereby begins flattening the tube, at the same time as the other roller begins the opposite operation. However, this design is not able to prevent considerable positive pressure and compressive pulsations from being produced in the pumped fluid.
During fluid entrance into the entrance section, the rearmost roller successively displaces some of the fluid in the tube. The process corresponds to the process in a hydraulic pressure cylinder where a piston displaces fluid in a cylinder. The length of the entrance section will thereby correspond to the stroke of the piston.
However, the presence of the oblique ramps in the pump known from the above U.S. patent results in a short length of stroke with a quick displacement of the fluid under the roller. This quick displacement of fluid causes the creation of a compression wave in the tube.
The reverse process takes place in the exit section where the foremost roller quickly is pulled out of engagement with the tube and thereby leaves a space which just as quickly is filled with affluent fluid.
The oblique ramps thus add to the inclination of positive pressure and compressive pulsations being produced in the pumped fluid.
U.S. Pat. No. 3,787,148 is furthermore based on the condition that the process in the entrance section is outbalanced by the process operating in opposite directions in the exit section. That the fact is different in reality is due to the circumstance that the rearmost roller begins pressing in on a full tube while the foremost roller begins to disengage a flat tube and that the displaced fluid volume is increasing concurrently with the tube being compressed and decreasing concurrently with the compression being discontinued. When the foremost tube starts opening, there is furthermore a pressure difference over the opening corresponding to the difference between the feed pressure and the suction pressure. Such a pressure difference is not present at the simultaneous closing of the tube by the rearmost roller.
In this way, neither the conventional peristaltic pumps nor the pump known from U.S. Pat. No. 3,787,148 can function without considerable rises in pressure occurring in the fluid in a cyclically repetitive way.
U.S. Pat. No. 4,564,342 and U.S. Pat. No. 5,470,211 disclose other peristaltic pumps which both are arranged in such a way that the flexible tube is always fully occluded by one roller. Therefore these pumps are only able to operate with some compressive pulsations.
Thus, there is a need for new peristaltic pumps which avoid these problems, and the present invention now provides devices which satisfy this need.
The present invention provides a peristaltic fluid pump of the kind mentioned in the opening paragraph, that is arranged to pump a fluid with smaller pressure differences and compressive pulsations in the fluid than previously known. The pump also is arranged to be able to pump with a discharge pressure that is constant during an entire pump cycle.
These features and benefits are obtained in a peristaltic fluid pump having a suction side and a pumping side and which comprises: a pump housing having a mainly arcuate support surface, a flexible tube extending along the support surface, a rotor having two opposite rollers for during operation rolling over the flexible tube along an entrance section where the tube successively is compressed, a pumping section extending across an angle having an arc of less than 180xc2x0, an exit section where the compression successively is ended, and means for rotating the rotor during operation. Advantageously, the entrance and exit sections include an idling zone that does not provide a pumping action, and a pumping zone that does provide a pumping action. Also, the arcuate support surface is constructed in such a way that the two opposite rollers do not operate in synchronous phase opposition during operation, and the entrance section end and the exit section beginning are arranged at an angle that is smaller than 180xc2x0.
Preferably, the arcuate support surface is arranged in such a way that the rearmost roller enters into the pumping zone of the entrance section sufficiently to build up pressure between the rollers to a level which is the same as the pumping pressure, and the pumping zone of the exit section terminates at an angle of an arc (xcex2) of more than 180xc2x0 in relation to the pumping zone of the entrance section. It is beneficial for the arcuate support surface to extend along a curve such that fluid displacement during the passage of the rollers along the entrance and exit sections of the tube changes linearly during operation.
In one embodiment, the pump includes pressure means for elastically pressing against the tube outside of the engagement of the rollers to maintain the tube in a predetermined shape. Preferably, the pressure means comprises springs, such as disc springs, placed on opposite sides of the tube.
In other embodiments, the pump can include a device for affecting the tube with a spring power in the entrance section of the tube, or a device for affecting the tube with a spring power in a zone downstream of the exit section of the tube.